First and foremost, the narcissist's partner must have a deficient or a distorted grasp of her self and of reality. Otherwise, she (or he) is bound to abandon the narcissist's ship early on. The cognitive distortion is likely to consist of belittling and demeaning herself -- while aggrandising and adoring the narcissist.
The partner is, thus, placing herself in the position of the eternal victim: undeserving, punishable, a scapegoat. Sometimes, it is very important to the partner to appear moral, sacrificial and victimised. At other times, she is not even aware of this predicament. The narcissist is perceived by the partner to be a person in the position to demand these sacrifices from her because he is superior in many ways (intellectually, emotionally, morally, professionally, or financially).
The status of professional victim sits well with the partner's tendency to punish herself, namely: with her masochistic streak. The tormented life with the narcissist is just what she deserves.
(From the book "Malignant Self-love: Narcissism Revisited" by Sam Vaknin - Click on this link to purchase: http://www.narcissistic-abuse.com/thebook.html)